A new pathway for iron-sulfur cluster repair

细菌几乎栖息在地球上的每一个生态位，包括一些恶劣到许多其他形式的生命无法生存的环境。这种成功至少部分是由于细菌适应环境变化的能力，这种适应植根于它们根据外部和内部线索改变基因表达模式的能力。许多细菌监测的一个关键环境参数是氧浓度。我们对大肠杆菌（E. coli）特别感兴趣。它的一个显著特性是它能够在有氧和没有氧的情况下茁壮成长。要做到这一点，它必须极大地改变它的新陈代谢，但这有后果，因为没有氧气，与有氧气时相比，能量节约和生长的潜力是有限的。为了测试是否有氧气存在，大肠杆菌使用一种叫做FNR的蛋白质，它起到氧气传感器的作用。它有一种特殊的辅助因子，叫做铁硫簇，它与氧反应，以一种关闭FNR的方式。这涉及到集群从一种形式（称为[4Fe-4S]形式）到另一种形式（称为[2Fe-2S]形式）的转换。在“非状态”下，FNR处于[2Fe-2S]形式，不能与DNA结合以激活在缺氧条件下生长过程中使用的基因表达。当没有氧气时，铁硫簇保持[4Fe-4S]形式，蛋白质可以与DNA结合并激活在缺氧条件下生长所需的基因表达。在过去的几年里，我们研究了氧与FNR铁硫团簇的反应。这些研究揭示了该反应的复杂生物化学原理，以及FNR蛋白是如何成为极其敏感的氧传感器的。我们最近表明，在其“关闭状态”下，[2Fe-2S]簇与大多数其他先前表征的[2Fe-2S]簇辅助因子不同，这是由于它是通过[4Fe-4S]形式的氧化转化（可被认为是氧化损伤）形成的。这导致了一种不寻常的蛋白质修饰，其中来自初始[4Fe-4S]簇的硫化物在转化反应中附着在蛋白质上。这可以被认为是一种储存硫的形式，值得注意的是，当氧不再存在时，它可以用来修复星团，使其恢复到原来的[4Fe-4S]形式。在其他铁硫簇蛋白中也发现了不寻常的[2Fe-2S] FNR簇，这提高了通过这种机制修复氧化损伤的[4Fe-4S]簇的可能性。使用广泛的方法，我们建议进一步研究不寻常的[2Fe-2S]簇的形成和[4Fe-4S]簇的修复，以确定其机制及其在体内作为氧化损伤铁硫簇修复的先前未被认识的途径的重要性。

Bacteria inhabit almost every environmental niche on Earth, including some that are so harsh that many other forms of life cannot survive. This success is at least in part due to the ability of bacteria to adapt to changes in environment, and this adaptation is rooted in their capacity to alter patterns of gene expression in response to external and internal cues. A key environmental parameter that is monitored by many bacteria is oxygen concentration. We are particularly interested in the bacterium Escherichia coli (E. coli). One of its remarkable properties is that it is able to thrive both in the presence and absence of oxygen. To do this it has to dramatically alter its metabolism, but this has consequences because without oxygen the potential for energy conservation and growth are limited compared to when oxygen is present. To test whether oxygen is present E. coli uses a protein called FNR, which acts as an oxygen sensor. It has a special co-factor called an iron-sulfur cluster that reacts with oxygen in a way that switches FNR off. This involves the conversion of the cluster from one form (called the [4Fe-4S] form) to another (called the [2Fe-2S] form). In the 'off-state' FNR is in its [2Fe-2S] form and cannot bind to DNA to activate expression of genes that are used during growth in the absence of oxygen. When there is no oxygen the iron-sulfur cluster remains in the [4Fe-4S] form and the protein can bind to DNA and activate expression of genes that are needed for growth in the absence of oxygen.For the last few years we have studied the reaction of oxygen with the FNR iron-sulfur cluster. These studies have revealed the complex biochemistry of the reaction and how this makes the FNR protein an exquisitely sensitive oxygen sensor. We recently showed that in its 'off-state' the [2Fe-2S] cluster is different from most other previously characterised [2Fe-2S] cluster cofactors and that this is a result of the fact that it is formed through oxidative conversion (which can be thought of as oxidative damage) of the [4Fe-4S] form. This results in an unusual modification of the protein in which sulfide from the initial [4Fe-4S] cluster is attached to the protein during the conversion reaction. This can be thought of as a form of stored sulfur and, remarkably, this can be used to repair the cluster back to its original [4Fe-4S] form when oxygen is no longer present. The unusual [2Fe-2S] cluster of FNR has also been discovered in other iron-sulfur cluster proteins, raising the possibility that repair of oxidatively damaged [4Fe-4S] clusters via this mechanism might be widespread.Using a wide range of approaches, we propose to investigate further the formation of the unusual [2Fe-2S] cluster and the repair of [4Fe-4S] cluster to determine the mechanism and the importance of this in vivo as a previously unrecognised pathway for repair of oxidatively damaged iron-sulfur clusters.